main () {
  // 如果想要声明一个变量有可能为空
  // String? xxx = null
  String? name = "zhangsan";
 name = null;
 String? name2;

 int? count = 0;
 count = null;
 print("名字的长度是 ${name?.length}");
 print("名字的长度是 ${count?.abs()}");

}